In U.S. Pat. No. 3,864,456 assigned to the assignee of this application, there is described a process for the production of chlorine dioxide which involves the reduction of sodium chlorate in an aqueous acid reaction medium which is maintained at its boiling point at the absolute pressure thereon while a sub-atmospheric pressure is applied to the reaction zone in which the reaction medium is located. The boiling temperature is greater than about 30.degree. C. and below the temperature above which substantial decomposition of chlorine dioxide occurs, preferably about 60.degree. to about 80.degree. C.
The reducing agent is chlorine ions provided by added sodium chloride, hydrogen chloride or a mixture of the two while the acid is provided either wholly by sulphuric acid in the case where sodium chloride alone is used as the source of chloride ions or partly by sulfuric acid and partly by hydrochloric acid in the case where hydrogen chloride along or in admixture with sodium chloride is used as the source of chloride ions.
The process operates on a continuous basis with reactants being continuously fed to the reaction medium and a gaseous mixture of chlorine dioxide, chlorine and water vapor being continuously removed from the reaction zone. The reaction medium is maintained at a total acid normality in the range of about 2 to about 4.8 normal and anhydrous neutral sodium sulphate is continuously crystallized from the reaction medium, once saturation is reached after start up. The crystallized sodium sulphate is removed from the reaction zone on a continuous or intermittent basis.
The just-described chlorine dioxide-producing process is commercially advantageous in that chlorine dioxide generation, water evaporation and by-product crystallization occur in the same vessel, anhydrous neutral sodium sulphate is formed directly, and a low total acid normality is employed. These and other qualities have led to wide commercial acceptance and implementation of the process.
It is known from U.S. Pat. No. 3,563,702 to increase the efficiency of production of chlorine dioxide by the above-described process from about 90 to 92% to about 97 to 98% by the use of certain catalysts, and silver salts commonly are used in commercial practice.
By the term "efficiency" is meant the degree of conversion of sodium chlorate fed to the reaction medium to product chlorine dioxide. The chlorine dioxide-producing reaction is represented by the following equation (1): EQU NaClO.sub.3 +NaCl+H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 +1/2 Cl.sub.2 +H.sub.2 O+Na.sub.2 SO.sub.4 ( 1)
There is a competing reaction which produces no chlorine dioxide and this reaction is represented by the following equation (2): EQU NaClO.sub.3 +5NaCl+3H.sub.2 SO.sub.4 .fwdarw.3Cl.sub.2 +3H.sub.2 O+3Na.sub.2 SO.sub.4 ( 2)
The efficiency of the process, therefore, is the extent to which the reaction of equation (1) can be made to predominate over the reaction of equation (2).
Since any decrease in efficiency of the process means that lesser quantities of sodium chlorate are converted to the desired chlorine dioxide product, and since sodium chlorate and silver salts are expensive raw materials, it is desirable to maintain the efficiency at as high a level as possible at all times. A number of factors can affect the efficiency of the process, mainly catalyst concentration and, to a lesser degree, mole ratio of chlorate ion to chloride ion in the reaction medium and temperature of the reaction medium.
In continuous plant operations, manual determinations of efficiency are effected to ensure operation at the desired efficiency level, any decrease in efficiency usually being compensated for by the addition of further quantities of catalyst, usually a silver salt, to the generator. In the event that the addition of further catalyst does not produce the desired result, then the operator must attempt to locate the source of the problem, which may be difficult to achieve.
Two types of efficiency determinations may be made, one based on the quantity of chlorate consumed and the quantity of chlorine dioxide produced. This determination provides an efficiency expressed as a percentage, signifying the percentage of one mole of chlorate which is reacted by equation (1) to form chlorine dioxide. This efficiency determination is only rarely made when a mass balance of the system is required, the chlorate feed and chlorine dioxide product values being monitored over the time interval and the determination being made from these monitored values.
The other manual determination which is made is the Gram Atom Percent Chlorine Dioxide (GA% ClO.sub.2) value of the product stream. GA% ClO.sub.2 is determined from the following equation (3): ##EQU1## by determining the chlorine atom present in the product gas stream as chlorine dioxide and chlorine.
The GA% ClO.sub.2 value is an accurate representation of the chemical efficiency and 100% efficiency is reached at a GA% ClO.sub.2 value of 50%. This value is a valid determination of efficiency for the above-described process since chlorine is produced along with the chlorine dioxide and is present in the product gas stream, in contrast to some chlorine dioxide-producing processes wherein the chlorine is reduced in situ to form chloride ions and the GA% ClO.sub.2 value would not represent a true indication of efficiency.
The determination of efficiency as the GA% ClO.sub.2 value is simpler to effect than the determination based on measurements of chlorate consumed and chlorine dioxide formed, requiring the withdrawal of a sample of product gas and analysis of the sample, generally by wet chemical means, to determine chlorine dioxide and chlorine contents. The GA% ClO.sub.2 determination, however, is made at widely-spaced time intervals typically varying from once a shift to once a week. However, the product gas stream is at a high temperature and under subatmospheric pressure and operator skill is required to collect a representative sample for analysis.
Owing to the necessity for skilled operation and the problem of representative sample collection, the GA% ClO.sub.2 efficiency value determined may be incorrect. Further, variations in efficiency between the periodic determinations are not compensated for. As a result, the overall efficiency of the chlorine dioxide producing process on a long term basis may well be, and usually is, less than the optimum, leading to less overall chlorine dioxide production and increased chemical and catalyst usage than the optimum.
As a result of variations in efficiency between efficiency determinations, not only is chemical usage less than optimum but also the concentration of the chlorine dioxide solution produced varies. This variation in chlorine dioxide concentration leads to variable pulp quality when the solution is used for bleaching.
Another important parameter of the process is the rate at which chlorine dioxide is produced, since this rate must be at least equal to the rate at which chlorine dioxide solution is consumed in the end use thereof. At the present time, a target production rate is set for the process, but little attempt is made to ensure that the production rate meets the requirements for product, which may vary from time to time.
In large volume chlorine dioxide generators, the system is less sensitive to changes in conditions, such as, chloride to chlorate mole ratio and catalyst concentration in the reaction medium and temperature, than in smaller volume generators having the same chlorine dioxide production capacity. In view of the fabricating costs of chlorine dioxide generators, which are generally constructed of titanium, the trend is to smaller volume generators, which leads, as noted above, to greater sensitivity of the process to parameter variations.
For greater overall efficiency and increased chlorine dioxide production and hence decreased raw material costs in the form of sodium chlorate and catalyst, and the production of more uniform product, it is necessary to provide a continuous accurate and rapid determination of efficiency, production rate and other parameters of the system, so that all variations affecting the chlorine dioxide production can be compensated for.